1. Field of the Invention
The present invention relates to a thin film transistor, a manufacturing method of the same, and an active matrix display apparatus. Particularly, the present invention relates to a thin film transistor formed on a metal substrate, a manufacturing method of the same, and an active matrix display apparatus including the thin film transistor.
2. Description of the Related Art
With the recent intensive development of highly sophisticated visualization and information society and rapid proliferation of multimedia systems, display apparatus such as LCDs (Liquid Crystal Displays) and organic EL (Electro Luminescence) displays become increasingly important. Such display apparatus have advantages such as low power consumption, low profile and lightweight and are thus widely used as a display of a portable terminal device or the like in place of CRT (Cathode Ray Tube) which have been dominant before.
As a driving method of pixels in such liquid crystal display apparatus and organic EL display apparatus, active matrix using TFTs (Thin Film Transistors) arranged in an array on a substrate is widely adopted. A TFT used in the active matrix display apparatus such as liquid crystal display apparatus typically has the structure that a source region, a drain region and a channel region are formed in a silicon layer that is deposited on a glass substrate. The display apparatus using a TFT array substrate does not have the flexibility in shape and thus it is not suited for a curved surface.
In order to increase the flexibility in shape of display apparatus, a technique of forming a TFT on a plastic substrate or a metal substrate has been proposed. However, because a plastic substrate has low heat-resistance, it is difficult to process a silicon layer to be formed on the plastic substrate to thereby complicate the manufacturing process. Therefore, the use of a metal substrate attracts attention in order to produce a shape-flexible display apparatus with a relatively simple manufacturing method. A flexible display apparatus using such a metal substrate is disclosed in Y. R. Luo et al., “The Electrical Characteristics of Low Temperature Polycrystalline Silicon Thin Film Transistor Fabricated on Steel Foil”, Proceedings of AM-LCD'05, (2005), p.231, and N. Young et al., “LTPS on Passivated Stainless Steel Substrates for AMOLEDs and Other Applications”, Proceedings of AM-LCD'05, (2005), p.239. Particularly, an organic EL display device is well suited for the use of a flexible metal substrate because it employs a solid-state light-emitting device.
As disclosed therein, each TFT formed on the metal substrate has substantially the same structure as a TFT formed on a glass substrate in related arts. The TFT is formed on an insulating protective film deposited on the metal substrate. The insulating protective film may be an inorganic or organic insulating film.
In a plurality of TFTs on the metal substrate of related arts, the voltage of a silicon layer to form each TFT is not fixed. Thus, the electrical characteristics of each TFT which is formed on the metal substrate are unstable. For example, an electron-positive hole pair occurs due to electron collision to thereby accumulate a charge in the silicon layer. As a result, a threshold voltage varies by the effect of back-gate bias. The use of a TFT with such unstable characteristics leads to degradation of display characteristics. The degradation of TFT characteristics is significant when using a low-temperature polysilicon TFT. Further, because the organic EL display apparatus employs current drive system, it requires severe characteristics for a TFT.
It is possible to fix a common voltage of TFTs to the voltage of a metal substrate by using the metal substrate as a common voltage line. However, direct connection of the metal substrate and a silicon layer of a TFT promotes alloying and interdiffusion at the interface between the metal substrate and the silicon layer. This causes the contamination of a channel region by substance of the metal substrate to degrade the TFT characteristics, which leads to a decrease in the reliability of an active matrix display apparatus.
In an active matrix display apparatus, circuits including TFTs are formed on an insulating film that is deposited on the metal substrate. A line for providing a common voltage of TFTs is thus required. With an increase in load due to an increase in the number of pixels, the reduction of resistance of the common voltage line becomes more important. For example, a self-luminous active matrix luminous display apparatus such as an organic EL display apparatus includes a pixel electrode for transmitting a picture signal formed on the circuits, a counter electrode placed face to face with the pixel electrode with a luminous layer interposed therebetween, and so on. A line is also required for voltage fixation and current supply to the counter electrode, and the reduction of resistance of the line is important. Although it is possible to fix a voltage by bringing the counter electrode of the active matrix luminous display apparatus into connect with the metal substrate, a transparent electrode used for the counter electrode is hard to make an electrical connection with the metal substrate. Further, the problem of alloying and interdiffusion exists also at the interface between the metal substrate and the line to provide a common voltage. This causes a decrease in the quality of display apparatus.